1. Field of the Invention
The present invention relates to electronic circuits, and, in particular, to signal generators such as voltage-controlled oscillator circuits.
2. Description of the Related Art
For many electronic applications, an oscillator circuit is employed to generate a periodic oscillating waveform. An oscillator circuit may be implemented with a tuned amplifier having positive feedback from the amplifier's output terminal to its input terminal, which design takes advantage of the instability possible in circuits having such a feedback loop. Oscillator circuits are designed with instability such that there is a frequency at which the loop gain is real and greater than one. Once drive voltage and current are applied to the circuit, the oscillator output signal oscillates between the maximum and minimum values of the amplifier output, thus generating the periodic output signal having a frequency related to the loop gain. Since a tuned amplifier is employed, the oscillator's loop-gain frequency is, to a great extent, determined by the inductance (L) and capacitance (C) values used to tune the amplifier (i.e., the “tank” circuit). A voltage-controlled oscillator (VCO) is a circuit that generates a periodic output signal with frequency based on an input voltage level. In some VCOs, a varactor diode is employed since the space-charge capacitance of the varactor changes as a function of control voltage (e.g., reverse biasing voltage), thus changing the capacitance of the tank circuit. Many different circuit configurations are known in the art to implement a VCO.
As VCO output frequencies increase for radio frequency (RF) applications (e.g., above 1 GHz), many prior-art VCOs implemented within an integrated circuit (IC) employ an accumulation-mode varactor. For example, a common n-well structure for an accumulation-mode varactor may be an n-channel MOS FET fabricated in an n-well (or a p-channel MOS FET fabricated in a p-well). Alternatively, the accumulation-mode varactor may be a common n-well bi-polar transistor. The capacitance of the accumulation-mode varactor is formed from the combined capacitance of the oxide layer and depletion layer. The capacitance of the accumulation-mode varactor changes as the reverse bias voltage applied across the varactor changes the varactor's state between deep accumulation and strong depletion of charge in the semiconductor layers.
VCO circuits commonly employed in the prior art comprise a single-ended control voltage (VC) drive circuit to provide the varying bias voltage across the varactor. Such single-ended control voltage is applied to one terminal of the varactor, while the other terminal is generally coupled to a supply voltage (VDD) or AC-coupled through an inductor or capacitor to VDD. The single-ended control voltage sets the variable capacitance of the varactor. Setting the variable capacitance of the varactor, in turn, tunes the LC-tank circuit to the desired operating frequency f.
FIG. 1 shows an integrated differential LC-VCO 100 of the prior art as may be implemented within an integrated circuit and operating with an output frequency above 1 GHz. In LC-VCO 100, two accumulation-mode varactors 101 and 102 are coupled back-to back in a common n-well configuration, the common n-wells of varactors 101 and 102 driven at node N1 by the DC control voltage VC. The other terminal of each of varactors 101 and 102 (shown at nodes N2 and N3, respectively) is coupled through a corresponding resistor R to the supply voltage VDD. For LC-VCO 100 of FIG. 1, the output voltages at corresponding terminals of varactors 101 and 102 (at nodes N2 and N3) are AC-coupled through capacitors 104 and 105 to nodes N4 and N5 as output voltages VO− and VO+. One skilled in the art would recognize that the output voltages from varactors 101 and 102 do not necessarily have to be AC-coupled. Inductors 106 and 107 are coupled between 1) corresponding nodes N4 and N5 and 2) the supply voltage VDD. Inductors 106 and 107 may be coupled directly between the supply voltage VDD and nodes N2 and N3 if the output voltages VO− and VO+ are not AC-coupled.
Output voltages VO+ and VO− are driven through a differential amplifier formed from cross-coupled MOS FETs M1 108 and M2 109, with M1 108 and M2 109 biased, as known in the art, via the current mirror of MOS FET M3 110, MOS FET M4 111, and current source 112. Positive feedback for the differential amplifier is generally through the LC-tank formed between nodes N4, N5, and VDD. The resonant frequency f of LC-VCO 100 is determined by the LC-tank circuit (i.e., f=1/(2π√{square root over (LC)})). For the LC-tank, L is the inductance of the circuit generated from the combination of inductors 106 and 107, and C is the capacitance formed from the combination of 1) the varying capacitances of varactors 101 and 102, 2) the capacitances of capacitors 104 and 105, 3) the capacitances of the differential cross-coupled MOS FETs M1 108 and M2 109, and 4) various IC parasitic capacitances.
The single-ended control voltage (e.g., VC of FIG. 1) sets the variable capacitances of the varactors. However, use of a single-ended control voltage drive circuit provides poor common-mode noise rejection. One approach is to increase common-mode noise rejection with AC-coupling of the output voltages, such as described with respect to FIG. 1. AC-coupling adds capacitance, which reduces the amplitude of the oscillation waveform across the varactor by the capacitance divider ratio. Therefore, AC-coupling reduces phase noise effects in output voltages at the expense of lower frequency-tuning range and lower VCO gain. This, and related techniques for improving common-mode noise rejection, are described in greater detail in F. Svelto and R. Castello, “A 1.3 GHz Low-Phase Noise Fully Tunable CMOS LC VCO”, IEEE Journal on Solid State Circuits, Vol. 35, No. 3, March 2000, incorporated herein by reference.
In addition, accumulation-mode varactors have a voltage-capacitance curve in which a majority of the variation in capacitance occurs between −1 to +1 volts of change in bias voltage across the varactor. However, bias of the varactor with a single-ended control voltage might not vary capacitance over the entire voltage-capacitance curve. As illustrated in FIG. 1, prior-art differential LC-VCO circuits connect one side of each varactor (shown at nodes N4 and N5) directly to a terminal of the inductor which sets the DC bias point of the varactor to the DC value appearing at the other terminal of the inductor, which DC value is often the positive power supply VDD. Thus, prior art VCOs either i) utilize only half of the variable capacitance range of each varactor or ii) employ additional circuitry that allows for driving the varactors above and below the DC bias point.
One method of employing the entire capacitance range of each varactor is described in U.S. Pat. No. 6,469,587, entitled “Differential LC Voltage-Controlled Oscillator,” filed on Dec. 4, 2000, to Scoggins, which is incorporated herein by reference. Scoggins describes a voltage-controlled oscillator (VCO) that includes a pair of varactors that are coupled in a back-to-back configuration and that are driven by a differential control voltage having positive and complementary control-voltage components to generate an output oscillation waveform. A voltage converter is employed to amplify and shift the positive and complementary control voltage components, with respect to a VCO source voltage, to generate the intermediate differential control voltage. The output signal of the VCO is tuned, in frequency, by setting a VCO tank inductance and varying a VCO tank capacitance in accordance with the intermediate differential control voltage. The VCO LC-tank capacitance includes the capacitance of the back-to-back varactors that varies in accordance with a drive voltage across each varactor. The drive voltage is formed from the intermediate control voltage by applying one of the intermediate control-voltage components to the node formed where the corresponding back-to-back varactors are coupled, and by applying the other intermediate control-voltage component to a node to which the other terminal of each varactor is coupled.